Specifically I want to be able to detect that an application is running and disallow the user from continuing the install process (via disabling the Next button, etc.)
Seems like there's no way to do this without writing a custom script, but this seems like such a basic utility that I feel like I'm overlooking something.
Thanks in advance!
See How to stop a running process during an MSI based un-install?
You can use pskill to force a process to exit, or the windows command TASKKILL /IM someprocess.exe to ask the application to exit nicely.
If you want to detect and disable a button, then you're going to have to write a custom script. Windows Installer will automatically detect if files you're trying to update are in use by an application and prompt the user anyway. More commonly the solution is to just install, then schedule a reboot with configuration commands to run on reboot when you know the other app won't be running.